The present invention relates to the field of electrophotography and, more particularly, to a process for enhancing the contrast and density of an image recorded on an electrophotographic plate or recording element.
The conventional electrophotographic process has inherently lower gain than the silver halide photographic process. A low exposure in a conventional electrophotographic process results in a low amplitude differential voltage pattern on a photoconductor, and when developed with conventional toner, the resulting toned image has correspondingly low density and contrast. It has been a longstanding goal to increase the gain of the electrophotographic process so that higher density images may be produced from low exposures. This goal is of particular interest in applications such as diagnostic xeroradiography where the exposing x-rays pose a potential health threat to the patient, and the lowest exposure possible is desired.
In addition to conventional xeroradiographic processes, there are other electrophotographic processes that produce relatively weak differential patterns of voltage, charge, current, or conductivity and for which increases in gain or photographic speed are desirable. Such electrophotographic processes include, for example, photoelectrophoresis, as disclosed in U.S. Pat. No. 4,361,636; ionography, as disclosed in U.S. Pat. No. 4,070,577; and ion projection, as disclosed in U.S. Pat. No. 4,338,614.
It has been proposed to increase the gain of an electrophotographic system, particularly a xeroradiographic system, by amplifying a low amplitude differential voltage image produced by a low x-ray exposure. See, for example, U.S. Pat. No. 3,981,727. Here, amplification of a low amplitude differential voltage pattern is achieved by first developing such pattern with an opaque toner. The photoconductor, with toner image in place, is again uniformly charged and then uniformly illuminated to reexpose the photoconductor using the toned image as a mask. The reexposed image is then further developed by applying additional toner to increase the density range of the image. While this amplification technique is theoretically capable of producing a threefold or fourfold increase in image density, it does so at the expense of requiring repetitive charge and exposure steps.